1. Field of the Invention
The present invention relates to a release agent used for development of latent images such as static charge images in the process of forming images by electrophotography or the like, to a toner containing the release agent, and to a method for manufacturing the same.
2. Description of the Related Art
In an image forming apparatus for forming images by electrophotography, after forming static charge images, for example, on the surface of an image support such as an electrophotographic photoreceptor (hereinafter also referred to simply as “photoreceptor”) by various apparatus, a toner is supplied to develop static charge images and resultant toner images are transferred and fixed on a recording medium such as paper to thereby form images. The toner used for the development of the static charge images (hereinafter referred to as “toner for use in static image development”) comprises a resin having a binding property referred to as a binder resin, to which additives such as a colorant, charge controller, etc. are dispersed.
In the recent years, to attempt improvement of image quality, reduction in particle size of toner has been promoted. For example, a toner having an average particle size by volume as small as about 3 to 8 μm has been used. For the manufacture of the toner, a so-called pulverization method has generally been used in which a binder resin, a colorant, etc. are melted and kneaded together and thereafter the resultant kneaded mass is pulverized to obtain a toner. Such a method is disclosed in Japanese Unexamined Patent Publication JP-A 2004-361817, for example. However, the pulverization method involves a problem that the particle shape becomes indefinite as the particle size of the obtained toner is smaller to extremely worsen the powder fluidity. In a case where the powder fluidity of the toner is poor, the toner cannot be supplied stably to the surface of the photoreceptor during development to result in a problem of causing image defects such as unevenness in the density.
Further, the pulverization method involves a problem that it is difficult to make the particle size of the toner uniform and the charging performance of the obtained toner becomes not uniform. In a case of forming images by using a toner of not uniform charging performance, charged amount becomes insufficient to generate a toner not transferred to a transferring material during transfer to the transfer material thereby causing lowering of the image density. For improving the uniformity of the charging performance of the toner in the pulverization method, while it is necessary to conduct classification after pulverization of the kneaded mass to narrow the range for the grain size distribution of the toner, classification results in another problem of lowering the toner yield and increasing the manufacturing cost.
As a method of overcoming the problems described above in the pulverization method, a wet method has been studied. As described in JP-A 2004-361817, the wet method for obtaining a toner includes, for example,
(i) a suspension polymerization method in which monomers of a binder resin dispersed by a suspension stabilizer in water are polymerized under the presence of a colorant and the colorant is incorporated in the resultant binder resin particles,
(ii) a coagulation method by emulsion polymerization in which an aqueous dispersion of resin particles obtained by emulsion polymerization of monomers of a binder resin and an aqueous dispersion of a colorant are mixed to form coagulated particles and the resultant coagulated particles are fused under heating,
(iii) a phase-inversion emulsification method in which a water dispersible resin and a colorant are dissolved or dispersed in an organic solvent, a neutralizing agent for neutralizing dissociation groups of the water dispersible resin and water are added thereto under stirring to form resin droplets incorporating the colorant, etc. and the resultant resin droplets are subjected to phase-inversion emulsification,
(iv) a dissolution suspension method in which a toner material containing a binder resin and a colorant is dissolved or dispersed in an organic solvent to which the binder resin is soluble, the obtained solution or liquid dispersion is mixed with a liquid dispersion of an inorganic dispersant, for example, a less water soluble alkaline earth metal salt such as calcium phosphate or calcium carbonate, the resultant is granulated and then the organic solvent is removed therefrom, and
(v) an emulsification and dispersion method in which a binder resin, a colorant, etc. are dissolved or dispersed in a water insoluble organic solvent to which the binder resin is soluble, the obtained solution or liquid dispersion is emulsified and dispersed in an aqueous liquid dispersion and then the organic solvent is removed therefrom.
However, the methods (i) to (v) involve the following problems. For example, the polymerization methods such as (i) suspension polymerization method and (ii) coagulation method by emulsion polymerization involve a problem that the resin usable as the binder resin is restricted to vinyl polymers capable of being formed by radical polymerization since polymerizing reaction is conducted in water. Considering the fixing property of the toner and the transparency thereof in a case of use as a color toner, use of a polyester resin is more preferred to that of the vinyl polymer as the binder resin. As described above, it is preferred that the binder resin is properly selected in accordance with the characteristics required for the toner.
Further, the polymerization method also includes a problem that the monomer of the binder resin, the polymerization initiator, the suspension stabilizer, etc. remain in the inside of the toner particles to make the charging performance of the toner varied. In order to suppress the variation of the charging performance, while it is necessary to remove the residues, it is extremely difficult to remove the monomer, the polymerization initiator, and the suspension stabilizer intruded in the inside of the toner particles.
Further in the phase-inversion emulsification method (iii), the dissolution suspension method (iv), and the emulsion and dispersion method (v), since the organic solvent is used for dissolving or dispersing the binder resin, they require a solvent recovery device for coping with environmental problems to result in a problem that the scale of the manufacturing facility becomes huge. The methods of (iii) to (v) also involve a problem that the resin usable as the binder resin is restricted to water dispersible resins having dissociation groups or those resins soluble to organic solvents.
As a technique for solving the problems, the present applicant has previously proposed a method of melting and kneading a toner starting material such as a binder resin and a colorant, then mixing the resultant kneaded mass into an aqueous medium containing a dispersant and water, heating the aqueous medium mixed with the kneaded mass and stirring the same thereby dispersing the kneaded mass into the aqueous medium and producing particles of the kneaded mass, and separating the particles of the kneaded mass from the aqueous medium to obtain a toner (hereinafter also referred to as “melting emulsification method”) (for example in Japanese Unexamined Patent Publication JP-A No. 2005-165039 (pages 4, 8 to 9).
According to the melting emulsification method disclosed in JP-A 2005-165039 described above, since various resins can be used as the binder resin, a toner having a desired characteristic can be manufactured relatively easily. However, with a view point of more reliably obtaining a toner having the desired characteristic, the technique disclosed in JP-A 2005-165039 also leaves a room for the improvement.
In the image forming process, for fixing toner images to a recording medium, a so-called heat roller fixing method of fixing toner images by heating and pressing a recording medium on which toner images are transferred by a heat roller heated to a predetermined temperature thereby fixing toner images has been used frequently. In the heat roller fixing method, an offset phenomenon is liable to occur and, in order to prevent the occurrence of the offset phenomenon, a release agent such as a wax is added to the toner. The offset phenomenon is such a phenomenon that a portion of the toner is transferred from a material to be transferred to a heat roller during fixing and the transferred toner is re-transferred to the subsequent material to be transferred. The offset phenomenon includes a low temperature offset phenomenon and a high temperature offset phenomenon. The low temperature offset phenomenon is an offset phenomenon caused by the occurrence of disconnection of toner images when the toner is not melted sufficiently under heating by the heat roller. The high temperature offset phenomenon is such an offset phenomenon caused by overheating of the toner due to heating by the heat roller to lower the coagulation between the toners to each other and lower the fixing property of the toner images to the recording medium.
In recent years, along with increase in the image forming speed, heating time for the toner by the heat roller during fixing has tended to be shortened. Further, corresponding to the increasing demand for energy saving in the image forming apparatus, it has been required to lower the heating temperature of the heat roller. Accordingly, the low temperature offset phenomenon is liable to occur during fixing and it has been demanded for preventing the low temperature offset phenomenon.
As the release agent for prevent the low temperature offset phenomenon, those capable of fusing at a temperature lower than the softening temperature of the binder resin thereby lowering the melt viscosity of the toner, for example, a wax having a melting point lower than the softening temperature of the binder resin is used. In the melting emulsification method, since the kneaded mass softened by heating is dispersed in an aqueous medium by pulverizing the same under a shearing force by stirring, the aqueous medium mixed with the kneaded mass is heated preferably to higher than the softening temperature of the binder resin contained in the kneaded mass. However, in a case where the temperature of the aqueous medium is higher than the melting point of the wax contained in the kneaded mass, particularly, higher by 20° C. than the melting point of the wax, there is a possibility that the wax is detached from the kneaded mass. Further, since the wax has a poor compatibility with the binder resin, the wax may possibly be detached by the application of a force such as a shearing force on the boundary between the wax and the binder resin in granulating the kneaded mass.
In a case where the wax detached from the kneaded mass and liberated into the aqueous medium (hereinafter also referred to as “free wax”) remains in the toner, the free wax adheres to the surface of the image support such as a photoreceptor during development tending to cause a phenomenon referred to as filming in which the wax is fused in the film-like shape. In order to prevent the free wax from remaining, it is necessary to wash the toner particles over and over in the cleaning step after granulating to result in lowering of the productivity. Further, this also results in a problem of yielding a great amount of waste water by cleaning to increase environmental burden.
Further, for granulating the kneaded mass into a desired particle size, since it is necessary to provide a shearing force by stirring the aqueous medium containing the kneaded mass repetitively, this also results in a problem that the wax liberated in the granulating step adheres to the surface of the granulated toner particles. In a case where the free wax adheres in a great amount to the surface of the toner particles, since the melt-viscosity is lowered and the coagulation force between the toner particles to each other is lowered, the high temperature offset phenomenon tends to be caused. Further, since the amount of the wax contained in the toner is decreased by so much as the wax liberated during granulating, even when an appropriate amount of the wax is added while considering the visco-elasticity of the toner so that the low temperature offset phenomenon can be prevented, the visco-elasticity of the toner is deviated from the design due to the detachment of the wax possibly causing the low temperature offset phenomenon. Accordingly, the temperature range of the heat roller capable of fixing the toner (hereinafter referred to as “temperature range capable of fixing”) is narrowed and, depending on the case, it is put to a state where the offset phenomenon occurs at any temperature.
As described above, since various problems occur in a case where the release agent such as a wax detaches in granulating the kneaded mass, it has been desired for the method capable of preventing detachment of the wax from the kneaded mass.